Nowadays, in the fields of, for example, automobiles, home electrical appliances and building material industries, surface treated steel sheets which are manufactured by providing untreated steel sheets with corrosion resistance, in particular, galvanized steel sheets and galvannealed steel sheets (hereinafter, generically called “galvanized steel sheets”), are widely used. In addition, from the viewpoint of the improvement of fuel efficiency and collision safety, the application of high strength steel sheets to automobile bodies has been progressing in order to decrease the weight and increase the strength of the automobile bodies by decreasing the thickness of car body materials by increasing the strength of the materials.
The galvanized steel sheet described above is manufactured from a thin steel sheet, which is manufactured by hot-rolling a steel material and then by cold-rolling the hot-rolled steel sheet, by performing recrystallization annealing on the thin steel sheet in an annealing furnace constituting a part of a continuous galvanizing line, and then by performing a hot dip galvanizing treatment on the steel sheet, or optionally, by further performing a galvannealing on the galvanized steel sheet.
Although examples of heating methods of the annealing furnace constituting a part of a continuous galvanizing line include a DFF type (direct fire furnace type), an NOF type (non-oxidation furnace type) and an all radiant tube type, recently, the number of the lines consisting of an annealing furnace of an all radiant tube heating type has been increasing for the reasons of, for example, easy operation, low possibility of occurrence of pick up and low cost at which a coated steel sheet of high quality can be manufactured.
On the other hand, as an example of a method for increasing the strength of a galvanized steel sheet, a method in which a large amount of chemical elements such as Si and Mn which are effective for strengthening steel is contained in steel for the purpose of solid solution strengthening is widely used. However, there is a problem in that, since the chemical elements described above such as Si and Mn are elements which are easily oxidized, these chemical elements markedly decrease the wettability of hot dip zinc coating (zinc coatability). In view of this, in a conventional furnace of a DFF type and an NOF type, zinc coatability is improved by forming an internal oxidation layer inside the steel sheet by positively oxidizing the surface of the steel sheet in a heating furnace in order to form an oxidized film which mainly contains Fe while the concentration of the chemical elements such as Si and Mn at the surface of the steel sheet is suppressed, by reducing the surface in a reducing atmosphere, and by performing a coating treatment (refer to Patent Literature 1). However, there is a problem in that, since it is impossible to include an oxidation process in an annealing furnace of an all radiant tube heating type, it is impossible to achieve good zinc coatability of a steel sheet which contains chemical elements such as Si and Mn.
As an example of a method for solving the problem described above, Patent Literature 2 discloses a method in which a high strength galvanized steel sheet which contains a large amount of the chemical elements such as Si and Mn is manufactured by suppressing the internal oxidation of the surface layer of the steel sheet by controlling the dew-point of the atmosphere at a temperature in the range of 600° C. or higher in an annealing furnace to −40° C. or lower. However, although it is disclosed that a dew-point is controlled to −40° C. or lower in this method and that a specific method for achieving the specified dew-point is to absorb and remove the moisture in the furnace with an absorbent, it is difficult to steadily keep the dew-point of an atmospheric gas in the furnace at −40° C. or lower. Therefore, there is a problem in that it is impossible to steadily manufacture a high strength galvanized steel sheet having excellent zinc coatability by only using the method disclosed in Patent Literature 2.
Incidentally, as an example of a method for controlling the dew-point temperature of an atmospheric gas in a furnace, Patent Literature 3 discloses an annealing furnace, in which thin metal strips which are continuously fed into the furnace are directly heated and cooled in a reducing atmosphere, the furnace consisting of a circulation system in which the gas in the furnace is discharged to the outside of the furnace through a ventilation pipe which is placed at the boundary of an interior refractory and an exterior steel shell of the furnace wall, then the discharged gas is cleaned by removing moisture and oxygen from the gas, and then the cleaned gas is returned to the furnace. However, since the object of this method is to reduce the time required to start up a bright annealing furnace for stainless steel at the beginning of its operation or at the resumption of its operation after repair, there is no mention of the improvement of the zinc coatability of the surface of a steel sheet. In addition, gas discharged from the inside of the furnace and the inlet side and outlet side of the furnace is fed into a cooling zone after the impurities of the gas have been removed but not directly into the furnace. Moreover, the atmospheric gas is not discharged from the inside of the furnace after the refractory has been dried (during operation). Furthermore, although the dew-point temperature of the discharged gas is observed, the dew-point temperature of the inside of the furnace is not observed. Therefore, the distribution of the dew-point temperature is left unknown.
In addition, Patent Literature 4 discloses a method for manufacturing a high strength galvanized steel sheet having Si content of from 0.2 mass % to 2.0 mass % by using a continuous galvanizing line consisting of a non-oxidation furnace, the method including controlling the thickness of an oxidized film on the surface of the steel sheet by adjusting the combustion air ratio of the non-oxidation furnace and the dew-point of the atmosphere of a reducing furnace in order to improve zinc coatability. However, although it is disclosed that a dew-point is controlled in this method, this is a method for improving zinc coatability by using oxidation-reduction reaction in the non-oxidation furnace and the reducing furnace, which means that this is not a method which can be applied to a furnace of an all radiant tube heating type in which the surface of a steel sheet is not positively reduced.
In addition, Patent Literature 5 discloses an annealing line of a reducing atmosphere for a metal strip, the line consisting of a circulating channel which consists of a refining device through which an atmospheric gas is circulated and in which moisture is adsorbed. However, the object of the refining device of this annealing line is to prevent defective coloring of the surface of the metal strip which occurs when the thickness of the strip is switched between thin and thick gauges, and not to improve zinc coatability. Moreover, the control of dew-point is not investigated.